1. Field of the Invention
The present invention relates to a belt drive apparatus and an image forming apparatus, and more particularly to a belt drive apparatus changing its belt conveyance speed according to the pressure contact/separating states to a body to receive the pressure contact and to an image forming apparatus equipped with the belt drive apparatus.
2. Description of the Related Art
A conventional image forming apparatus of an electrophotographic system, such as a printer and a copying machine, develops a toner image on a photoreceptor drum on the basis of image data and transfers the toner image onto paper. Then, the image forming apparatus fixes the toner image by thermo-compression bonding in its fixing apparatus, and thereby forms an image on the paper.
As the fixing apparatus, a fixing apparatus adopting a belt system, which is equipped with a belt drive apparatus, including a plurality of supporting rollers and an endless belt (hereinafter simply referred to as a belt), laid across between the rollers in a tensioned condition, has been known. This fixing apparatus of the belt system brings the belt into pressure contact with, for example, a fixing roller, including a heating heater therein, to form a nipping section, and fixes a toner image, while nipping and conveying paper at this nipping section.
It is known that the belt of a belt drive apparatus used in such a fixing apparatus deviates to one end direction in the width direction thereof (the direction perpendicular to the traveling direction of the belt) and meanders as the belt travels. If the meander advances, the belt slips off from the supporting roller and is broken. Accordingly, it is generally performed to amend the meanders of a belt by detecting the position of the belt end while the belt is travelling, and by tilting one of the supporting rollers on the basis of the detection result to perform steering control (see, for example, Japanese Patent Application Laid-Open No. 2000-34031).
FIG. 7 is a view showing an example of a detection unit to detect the position of a belt end.
As shown in FIG. 7, a detection unit 110 is equipped with a transmission type photosensors PS1-PS3, each composed of a light emitting element and a light receiving element, which are arranged to be opposed to each other. A shielding body (a belt 101 itself in FIG. 7) moves between the light emitting elements and the light receiving elements, accompanying the position variations of a belt end 101a, and thereby transmits/intercepts lights from the light emitting elements. Each of the photosensors PS1-PS3 outputs a detected signal of a high level (hereinafter referred to as H level) or a low level (hereinafter referred to as L level) according to a light quantity received by the light receiving element thereof, and a control section judges the position of the belt end 101a on the basis of these detected signals. It is supposed, here, that the photosensors PS1-PS3 output an H level detected signal when the light from a light emitting element is intercepted and outputs an L level detected signal when the light from a light emitting element is transmitted.
For example, if an H level detected signal is output from the photosensor PS1 and L level detected signals are output from the photosensors PS2 and PS3 in the detection unit 110, the control section judges that the belt 101 is in a normal position where no meander amendments are needed.
If H level detected signals are output from the photosensors PS1 and PS2 and an L level detected signal is output from the photosensor PS3 (deviation to left in FIG. 7), or if L level detected signals are output from all of the photosensors PS1-PS3 (deviation to right in FIG. 7), then the control section judges that the belt 101 is at a position where a meander amendment is needed. In this case, the control section amends the meander of the belt 101 by steering control.
When H level detected signals are output from all of the photosensors PS1-PS3, the control section judges that the belt 101 is in an abnormal position where breakage of the belt 101 can be caused. In this case, the control section stops the drive of the belt 101 in order to prevent the advance of the meander and the breakage of the belt 101.
A belt drive apparatus equipped with such a detection unit 110 performs a so-called chattering removing processing in order to remove the influences of chattering caused by mechanical vibrations and the influences of roughening (irregularities) of the belt end 101a. In the chattering removing processing, by performing software control, the output signals from the photosensors PS1-PS3 are sampled twice (a so-called twice-reading) in accordance with a predetermined timer, and an abnormal/normal position of the belt 101 is judged when the values sampled twice agree with each other.
If there is not a sufficient time for removing the influences of chattering and the like, however, false detection can arise in the aforesaid chattering removing processing. If the false detection frequently arises, the reliability of the apparatus is deteriorated.
FIG. 8 is a diagram showing judgment results of belt positions on the basis of output signals from the photosensor PS3. FIG. 8 shows a case where false detection signals (a)-(e) are output owing to the influences of chattering and the like.
As shown in FIG. 8, if the false detection signal (a) is output from the photosensor PS3 when a real belt position is a normal position, it is falsely judged that the belt 101 has entered an abnormal position, because the signal levels sampled twice agree with each other, both of the values being H level. In this case, the restart of the belt drive is to be instructed just after the stopping of the belt drive is instructed, but a predetermined time is necessary for restarting the belt drive that has once been stopped.
Furthermore, if the false detection signal (d) is output from the photosensor PS3 when the real belt position is an abnormal position, it is falsely judged that the belt 101 has returned to a normal position, because the signal levels sampled twice agree with each other, both of the values being L level. In this case, the stopping of the belt drive is to be instructed just after the restart of the belt drive is instructed, and the processing load of the control section uselessly increases.
On the other hand, even if the false detection signals (b), (c), and (e) are output from the photosensor PS3, no false detection results because the signal levels sampled twice do not agree with each other.
It may be possible to simply lengthen the chattering removing processing time (for example, thrice-reading and the like) in order to effectively remove the false detection signals owing to the influences of chattering and the like. In that case, however, the timing of judging that the belt 101 has entered an abnormal position from a normal position becomes late, and consequently it becomes impossible to surely prevent the breakage of the belt 101. Furthermore, the timing of judging that the belt 101 has returned from an abnormal position to a normal position becomes late, and it becomes difficult to restart the drive of the belt 101 early.
Furthermore, if the belt 101 travels being in pressure contact with a body to receive the pressure contact, as in a belt drive apparatus to be used for a fixing apparatus, the meandering speed of the belt 101 in a pressure contact state in which the body to receive the pressure contact and the belt 101 are in pressure contact with each other, and the meandering speed of the belt 101 in a separating state in which the body and the belt 101 are separated from each other, are different from each other (the meandering speed in the separating state is faster than that in the pressure contact state), but the chattering removing processing is similarly performed in both the cases. Consequently, it cannot be said that the abnormality of a belt position is suitably judged according to the pressure contact/separating states. As a result, there is the possibility that the breakage of the belt 101 is caused or false detection is frequently caused.
Furthermore, because the technique described in Japanese Patent Application Laid-Open No. 2000-34031 detects the position variations of the belt end by an analog way, the detection mechanism for detecting a belt position and sensors to be used are complicated. Hence the technique has the problem of the difficulty of the cost reduction thereof.